Re: [PATCH 7/7] tcg/ppc: Use prefixed instructions for tcg_out_goto_tb

[Nicholas Piggin]

On Mon Aug 7, 2023 at 12:13 AM AEST, Richard Henderson wrote:
> On 8/6/23 05:55, Nicholas Piggin wrote:
> > On Sat Aug 5, 2023 at 7:33 AM AEST, Richard Henderson wrote:
> >> When a direct branch is out of range, we can load the destination for
> >> the indirect branch using PLA (for 16GB worth of buffer) and PLD from
> >> the TranslationBlock for everything larger.
> >>
> >> This means the patch affects exactly one instruction: B (plus filler),
> >> PLA or PLD.  Which means we can update and execute the patch atomically.
> >>
> >> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> >> ---
> >>   tcg/ppc/tcg-target.c.inc | 76 ++++++++++++++++++++++++++++++----------
> >>   1 file changed, 58 insertions(+), 18 deletions(-)
> >>
> >> diff --git a/tcg/ppc/tcg-target.c.inc b/tcg/ppc/tcg-target.c.inc
> >> index 5b243b2353..47c71bb5f2 100644
> >> --- a/tcg/ppc/tcg-target.c.inc
> >> +++ b/tcg/ppc/tcg-target.c.inc
> >> @@ -2642,31 +2642,41 @@ static void tcg_out_goto_tb(TCGContext *s, int 
> >> which)
> >>       uintptr_t ptr = get_jmp_target_addr(s, which);
> >>   
> >>       if (USE_REG_TB) {
> >> +        /*
> >> +         * With REG_TB, we must always use indirect branching,
> >> +         * so that the branch destination and TCG_REG_TB match.
> >> +         */
> >>           ptrdiff_t offset = tcg_tbrel_diff(s, (void *)ptr);
> >>           tcg_out_mem_long(s, LD, LDX, TCG_REG_TB, TCG_REG_TB, offset);
> >> -
> >> -        /* TODO: Use direct branches when possible. */
> >> -        set_jmp_insn_offset(s, which);
> >>           tcg_out32(s, MTSPR | RS(TCG_REG_TB) | CTR);
> >> -
> >>           tcg_out32(s, BCCTR | BO_ALWAYS);
> >>   
> >>           /* For the unlinked case, need to reset TCG_REG_TB.  */
> >>           set_jmp_reset_offset(s, which);
> >>           tcg_out_mem_long(s, ADDI, ADD, TCG_REG_TB, TCG_REG_TB,
> >>                            -tcg_current_code_size(s));
> >> +        return;
> >> +    }
> >> +
> >> +    if (have_isa_3_10) {
> >> +        /* Align, so that we can patch 8 bytes atomically. */
> >> +        if ((uintptr_t)s->code_ptr & 7) {
> >> +            tcg_out32(s, NOP);
> >> +        }
> >> +        set_jmp_insn_offset(s, which);
> >> +        /* Direct branch will be patched by tb_target_set_jmp_target. */
> >> +        tcg_out_mls_d(s, ADDI, TCG_REG_TMP1, 0, 0, 1);
> >>       } else {
> >>           /* Direct branch will be patched by tb_target_set_jmp_target. */
> >> -        set_jmp_insn_offset(s, which);
> >> -        tcg_out32(s, NOP);
> >> -
> >> +        tcg_out32(s, B);
> >>           /* When branch is out of range, fall through to indirect. */
> >>           tcg_out_movi(s, TCG_TYPE_PTR, TCG_REG_TMP1, ptr - (int16_t)ptr);
> >>           tcg_out_ld(s, TCG_TYPE_PTR, TCG_REG_TMP1, TCG_REG_TMP1, 
> >> (int16_t)ptr);
> >> -        tcg_out32(s, MTSPR | RS(TCG_REG_TMP1) | CTR);
> >> -        tcg_out32(s, BCCTR | BO_ALWAYS);
> >> -        set_jmp_reset_offset(s, which);
> >>       }
> >> +
> >> +    tcg_out32(s, MTSPR | RS(TCG_REG_TMP1) | CTR);
> >> +    tcg_out32(s, BCCTR | BO_ALWAYS);
> >> +    set_jmp_reset_offset(s, which);
> >>   }
> >>   
> >>   void tb_target_set_jmp_target(const TranslationBlock *tb, int n,
> >> @@ -2674,20 +2684,50 @@ void tb_target_set_jmp_target(const 
> >> TranslationBlock *tb, int n,
> >>   {
> >>       uintptr_t addr = tb->jmp_target_addr[n];
> >>       intptr_t diff = addr - jmp_rx;
> >> -    tcg_insn_unit insn;
> >>   
> >>       if (USE_REG_TB) {
> >>           return;
> >>       }
> >>   
> >> -    if (in_range_b(diff)) {
> >> -        insn = B | (diff & 0x3fffffc);
> >> -    } else {
> >> -        insn = NOP;
> >> -    }
> >> +    if (have_isa_3_10) {
> >> +        tcg_insn_unit insn1, insn2;
> >> +        uint64_t pair;
> >>   
> >> -    qatomic_set((uint32_t *)jmp_rw, insn);
> >> -    flush_idcache_range(jmp_rx, jmp_rw, 4);
> >> +        if (in_range_b(diff)) {
> >> +            insn1 = B | (diff & 0x3fffffc);
> >> +            insn2 = NOP;
> >> +        } else if (diff == sextract64(diff, 0, 34)) {
> >> +            /* PLA tmp1, diff */
> >> +            insn1 = OPCD(1) | (2 << 24) | (1 << 20) | ((diff >> 16) & 
> >> 0x3ffff);
> >> +            insn2 = ADDI | TAI(TCG_REG_TMP1, 0, diff);
> >> +        } else {
> >> +            addr = (uintptr_t)&tb->jmp_target_addr[n];
> >> +            diff = addr - jmp_rx;
> >> +            tcg_debug_assert(diff == sextract64(diff, 0, 34));
> >> +            /* PLD tmp1, diff */
> >> +            insn1 = OPCD(1) | (1 << 20) | ((diff >> 16) & 0x3ffff);
> >> +            insn2 = PLD | TAI(TCG_REG_TMP1, 0, diff);
> >> +        }
> > 
> > B is a "patch class" word instruction as per CMODX in the ISA, which may
> > be patched to/from other instructions without a flush+isync sequence
> > betwen. So that part is okay, at least if you were just patching the B
> > word. But patching between the PLA and PLD I don't think is kosher per
> > ISA.
> > 
> > I struggle a bit with this part of the ISA, particularly with prefix
> > instructions (it only talks about patching 4 bytes at a time).
> > 
> > If we patch something it has to go through a patch instruction, which
> > is a direct branch, trap, or nop. I think that makes this non-trivial.
> > 
> > It could work if you only patched between B and PLD. B->PLD would have
> > to patch the suffix word first, possibly with an interleaving sync, and
> > then the prefix. PLD->B could just patch the B word.
> > 
> > How much would losing the PLA hurt?
>
> Really?  I can't imagine how some icache would see a torn prefixed insn given 
> an atomic 
> store (CMODX talks about prefixed instructions which "may be unaligned" -- 
> but what if 
> they are not?).
>
> But if patching an aligned prefixed insn isn't allowed, I would patch between 
> B and NOP, 
> leave the PLD alone on the fall-through path, and drop the PLA.

Hmm, even patching two different offset B instructions in some sequence
with a PLD would go against the ISA, because PLD is not a patch class
instruction. The only case you can patch those has to have a sequence of
exactly two instruction values. So I think you need the B first, and you
can patch that between any offset of B or a NOP to fall through to PLD.
NOP and B are both patch class, so any sequence of them is allowed.

Thanks,
Nick